In recent years, as costs of energy increase along with concerns about global warming due to consumption of fossil fuels to generate energy, there is an ever increasing need for more efficient lighting technologies. These demands, coupled with rapid improvements in semiconductors and related manufacturing technologies, are driving a trend in the lighting industry toward the use of light emitting diodes (LEDs) or other solid state light sources to produce light for lighting applications, as replacements for incandescent lighting and eventually as replacements for other older less efficient light sources. In addition, LEDs are non-toxic, unlike compact fluorescent bulbs, which contain trace amounts of harmful mercury.
To provide efficient mixing of the light from a number of sources and a pleasing uniform light output, Advanced Optical Technologies, LLC (AOT) of Herndon, Va. has developed a variety of lighting fixture configurations that utilize light from a number of solid state sources. By way of example, a variety of structures for AOT's lighting systems are described in US Patent Application Publications 2007/0138978, 2007/0051883, 2007/0045524, 2009/0295266, and 2009/0296368, the disclosures of which are incorporated herein entirely by reference.
An example of a solid state light emitter 10 is shown in FIG. 1. A light source 12, such as an LED, is provided on a substrate 14 in a cavity of a reflector 16. The surface of the reflector 18 reflects and concentrates light emitted from the light source 12. Lead wires 20 connect the light source to electrodes 22 to carry electrical current to power the light source 12. The electrodes 22 are connected to an external power source (not shown). Appropriate materials for the LED 12 can be selected so that the device emits blue light. An example of a spectrum 28 of a blue light LED is shown in FIG. 2. As shown in FIG. 2, the peak emission of the blue light LED is at a wavelength of about 450 nm.
LEDs are available in a variety of colors depending on the semiconductor material used to fabricate the LED. LEDs can be produced which emit infrared, visible, and ultraviolet light. Typically, an LED emits light in a relatively narrow wavelength, thus light of substantially a single color is produced. Thus, LED lighting can cast an undesirable, unnatural-looking color over an area when used for general lighting. A lighting apparatus producing substantially white light for general lighting purposes would, therefore, be desirable. Substantially white light can be produced by using a phosphor in the LED package. Phosphors absorb excitation energy then re-emit the energy as radiation of a different wavelength. For example, a blue light absorbing phosphor can emit yellow light. The combination of blue light emitted by the light source and yellow light emitted by the phosphor can produce a substantially white light. Substantially white light can also be produced by combining LED lights of different colors, such as combining LEDs emitting red, green, and blue light. Such lighting apparatuses require the use of three different types of LEDs in a single lighting apparatus. In addition, the light in a LED lighting fixture typically passes across a plurality of different surfaces, resulting in internal reflections which both reduce light intensity and increased yellowing of the light, as light reflected back into a region containing phosphor undergoes additional wavelength shifting.
In view of the above shortcomings of LED lighting, it would be desirable to provide white light from a lighting apparatus using only one type of solid state light source. These developments not withstanding, in this age of ever increasing concern over energy consumption, there is always a need for techniques to provide lighting applications that are energy efficient, but which also can generate a visibly pleasing light distribution.